Phospholemman (PLM) is the major plasmalemmal substrate for cAMP-dependent protein kinase (cAMPK) and protein kinase C (PKC). Canine and murine PLM are expressed at high levels in heart, skeletal muscle, and liver, and at low levels in breast, brain, lung, stomach, kidney, and colon (Palmer C et al (1991) J Biol Chem 266: 11126-11130; Moorman J R et al (1992) J Biol Chem 267:14551-14554). PLM is a membrane protein which consists of 72 amino acids and has a calculated molecular weight of 8409. The native protein has an apparent molecular weight of 15 kdal from polyacrylamide gel electrophoresis. A distinguishing feature of PLM is its highly basic nature, with a calculated isoelectric point of 9.7 (Palmer C et al, supra). PLM consists of an acidic extracellular amino-terminal domain, a single uncharged transmembrane domain, and an extremely basic cytoplasmic carboxy-terminal domain. The cytoplasmic domain contains consensus cAMPK and PKC phosphorylation sites. The phosphorylation of PLM by PKC and cAMPK is regulated by insulin and adrenaline, respectively (Walaas S et al (1994) Biochem J 304:635-640). PLM phosphorylation in cardiac muscle occurs after activation of either .alpha.- or .beta.-adrenergic receptors, and correlates with an increase in contractility (Lindemann J P (1986) J Biol Chem 261:4860-4867).
Expression of PLM in Xenopus oocytes injected with PLM mRNA coincides with the appearance of voltage-activated chloride currents (Moorman et al, supra). Immunoaffinity-purified recombinant PLM added to planar phospholipid bilayers produces unitary anion currents (Moorman J Ret al (1995) Nature 377:737-740). The high selectivity of the PLM channel for the sulfonic amino acid taurine suggests that PLM channels link signal transduction cascades to cell volume regulation. The investigators report that PLM is the smallest membrane protein known to form an ion channel (Moorman et al (1995), supra).
Mat-8, an 8-kDa transmembrane protein related to PLM, is expressed in murine breast tumor lines transformed by Neu or Ras oncoproteins. Morrison B W et al ((1994) Oncogene 9:3417-3426) proposed that Mat-8 is a marker of the cell type preferentially transformed by neu or v-Ha-ras oncogenes. A human Mat-8 homolog is expressed both in primary breast tumors and in breast tumor cell lines. Murine Mat-8 is also expressed in uterus, stomach, colon, and at low levels in virgin breast, ovary, lung, small intestine and thymus. In contrast to PLM, Mat-8 is not expressed in liver, heart or skeletal muscle, which suggests distinct cellular functions for the two molecules (Morrison B W et al (1995) J Biol Chem 270:2176-2182).
The extracellular and transmembrane domains of Mat-8 are homologous to those of PLM. However, the cytoplasmic domain of Mat-8 is unrelated to PLM and contains no consensus phosphorylation sites for PKC or cAMPK. Expression of Mat-8 in Xenopus oocytes induces voltage-activated chloride currents similar to those induced by expression of PLM (Morrison et al (1995), supra). However, direct ion channel formation by Mat-8 has not been reported. The ability of Mat-8 protein to induce chloride channel activity, together with its tissue distribution (discussed above), suggests that this protein may be involved in the regulation of transepithelial transport in tissues containing absorptive or secretory epithelia.
Additional proteins similar in structure to PLM and Mat-8 have been found to induce ion channel activity when expressed in Xenopus oocytes. Channel inducing factor (CHIF), found in colon and kidney, consists of a single transmembrane domain and exhibits 50% sequence similarity to PLM (Attali B et al (1995) Proc Natl Acad Sci U.S.A. 92: 6092-6096). Xenopus oocytes injected with CHIF mRNA exhibit K+ specific channel activity. Slow-activating voltage dependent potassium ion channel (IsK; Takumi T et al (1988) Science 242:1042-1045) is a single transmembrane domain glycoprotein present in epithelial cells, heart, uterus and lymphocytes (Attali B et al (1993) Nature 365:850-852). IsK induces both K+ and Cl- currents when expressed in Xenopus oocytes and HEK 293 cells. The accumulated evidence suggests that CHIF and IsK act as regulatory subunits of pre-existing channel complexes rather than as channels per se (Attali B et al (1995), supra; Ben-Efraim I et al (1996) J Biol Chem 271:8768-8771).
The sodium potassium ATPase (Na,K-ATPase) .gamma.-subunit, formerly known as the Na,K-ATPase proteolipid, is a small membrane protein that co-purifies with the .alpha.- and .beta.-subunits of Na,K-ATPase (Mercer R W et al (1993) J Cell Biol 121:579-586). The .gamma.-subunit is a small membrane protein consisting of 58 amino acids with a single transmembrane domain. This transmembrane domain is structurally related to the transmembrane domains of other PLM-like proteins. The .gamma.-subunit may act as a regulator of the ATP-dependent ion channel activity of Na,K-ATPase.
Discovery and molecular characterization of new members of the family of PLM-like proteins satisfies a need in the art by providing new opportunities to understand and modulate physiological processes including neurotransmitter release, transepithelial transport, membrane potential stabilization, signal transduction and cell volume regulation.